1. Field of the Invention
The present invention relates generally to the removal of soluble copper from aqueous effluents, and more particularly to a method for removing copper and copper-cyanide complexes from mining wastewater.
In conventional gold extraction techniques, a dilute aqueous solution of sodium cyanide is added to the finely crushed gold ore and the slurry is aerated to dissolve the metallic gold present as soluble aurocyanide ion. The aurocyanide ion is then treated with metallic zinc, which precipitates metallic gold. The remaining aqueous solution must then be treated to remove the extremely toxic cyanide before the solution is discharged to the environment. This may be accomplished by oxidizing the cyanide with hydrogen peroxide in the presence of cationic copper (2-10 ppm). If cationic copper is not added the oxidation of cyanide ion proceeds too slowly to be industrially practicable. While this treatment effectively removes the cyanide to very low concentrations, the residual cationic copper concentration is too high to be environmentally acceptable and must be removed prior to discharge. The copper present in the effluent after removal of the cyanide exists both as the free hydrated copper cation and as a copper-containing anionic complex with the cyanide ion, typically cuprocyanide [Cu(CN).sub.4 ].sup.-3 and possibly cupricyanide [Cu(CN).sub.4 ].sup.-2.
Theoretically, both cationic copper and the copper-cyanide complexes may be removed from solution by precipitation since, under certain obtainable conditions, they both form insoluble compounds. In practice, however, the presence of both species at very low concentrations inhibits both the formation of the insoluble compounds and their subsequent precipitation. Specifically, the formation of the insoluble compounds proceeds extremely slowly in the absence of a large excess of the reacting components; and even after the compounds are formed, they are present in such low concentrations that they are very slow to coalesce to form particles large enough to precipitate.
Conventional methods employed in chemistry are not effective when applied to highly dilute solutions, and for this reason ingenious procedures must be sought to remove the last traces of cationic and anionic copper when these species are present in only a few parts per million. It is the purpose of this invention to describe a process for removing extremely minute quantities of both anionic and cationic copper from solution.
2. Description of the Prior Art
U.S. Pat. No. 4,070,281 to Tagashira, et al., discloses the oxidation of cyanide-containing wastewater with cationic copper and an ammonium salt, the process leaving residual soluble cationic copper which must be removed. One method for effecting removal is the addition of ferrous chloride, iron powder or ferrous sulfate to precipitate the copper. U.S. Pat. No. 3,931,007 to Sugano, et al., discloses the addition of hydroxide ion to remove various metals, including copper. However, only incomplete removal of copper is achieved. Patterson, "Wastewater Treatment Technology," Ann Arbor Science Publishers, Inc., Ann Arbor, Mich. (1976), discusses the use of ferrous ion to remove copper under alkaline conditions. Other methods for copper removal are discussed in U.S. Pat. Nos. 3,817,306; 4,054,516; 4,072,605; 4,172,785; and Japanese Patent No. 85,569 and Russian Patent No. 528,265.